The present invention relates to a tunable laser source apparatus that can vary an oscillating wavelength used in the field of optical communications and precision measurements, and in particular, to a tunable laser source apparatus that can continuously sweep the oscillating wavelength over a wide band using an optical amplifying function as provided by a semiconductor laser, which covers a range of wavelengths over a wide band (hereafter referred to as an xe2x80x9cLDxe2x80x9d).
Various optical parts such as an optical fiber amplifier, an optical filter, and an optical isolator as well as each device constituting a transmission system which are all used for wavelength multiplexed communication must have their wavelength band characteristics measured.
Thus, a tunable laser source apparatus for a wide band is required which provides light of a predetermined wavelength.
In this case, the ability to continuously sweep the wavelength is desirable.
A tunable laser source apparatus called an xe2x80x9cexternal cavity laserxe2x80x9d has been spread which passes light from an optical amplifying element such as an LD which has a wide gain band, through a wavelength selecting element such as a diffraction grating which is arranged outside the element, to feed back light of a desired wavelength band, thereby causing laser oscillation within that wavelength band.
In this case, the most commonly used wavelength selecting element is a diffraction grating.
That is, a selected wavelength is varied by varying the angle of the diffraction grating relative to an incidence direction of light.
FIG. 8A is a view useful in explaining the configuration of a tunable laser source apparatus of this kind, that is, a typical external cavity laser using a diffraction grating.
In addition, FIGS. 8B, 8C, 8D, and 8E are views useful in explaining the principle of wavelength determination.
That is, an external cavity laser such as that shown in FIG. 8A comprises an LD 51 with an anti-reflection film (hereafter referred to as an xe2x80x9cAR coat) applied to one end surface 51a, lasers 52a and 52b, and a diffraction grating 53 arranged on the AR-coated end surface 51a. 
The diffraction grating 53 is capable of rotation and translation.
The diffraction grating 53 and the other surface (the end surface that is not AR-coated) of the LD 51b constitute an external cavity.
Such an arrangement of the diffraction grating 53 that light from the LD51 is diffracted directly to the LD51 by the diffraction grating 53, which receives the light, so that the light has a selected wavelength, is called a xe2x80x9cLittrow mountingxe2x80x9d.
Regardless of the use of the Littrow mounting, the oscillating wavelength of an external cavity laser including a wavelength selecting element is determined by two factors.
One of them is a wavelength that meets resonance conditions determined by the optical length of the entire resonator that causes laser oscillation.
In an optical resonator such as that shown in FIG. 8B, the optical length of the entire resonator (hereafter referred to as the xe2x80x9cresonator lengthxe2x80x9d) is denoted by L, the frequency of incident light is denoted by "ugr", the power of incident light is denoted by P0, and the power of emitted light is denoted by P1.
As is well known, when the light speed is denoted by c, the free spectral range (hereafter referred to as the xe2x80x9cFSRxe2x80x9d) is expressed by:
(FSR=c/(2L).
As shown in FIG. 8C, for each FSR, there are a plurality of resonance frequencies at which transmittance (the power of emitted light P1/the power of incident light P0) is maximized.
When a resonance frequency is n times as large as the FSR, this frequency is called an xe2x80x9cn order modexe2x80x9d.
Here, a wavelength corresponding to such a resonance frequency is called a xe2x80x9cresonance wavelengthxe2x80x9d.
The other is the distribution of a gain with its band limited by a diffraction grating such as that shown in FIG. 8D or a general wavelength selecting element.
If the optical amplifying element such as the LD which has a gain over a wide band is used, the gain of the diffraction grating within a selected wavelength band is constant.
Thus, the distribution of the gain with its band limited may be considered to be identical to a selected wavelength spectrum of the diffraction grating.
Accordingly, a peak wavelength of the selected wavelength spectrum is hereafter simply called a xe2x80x9cselected wavelengthxe2x80x9d.
Then, one of the modes which is located at a frequency having the highest gain starts to oscillate as shown in FIG. 8E.
In general, the selected wavelength does not equal the oscillating wavelength.
FIGS. 9A, 9B, 9C, 9D, and 9E shows variations in oscillating wavelength observed when the change rates of the resonance and selected wavelengths are not equal.
When a resonator length L and an incident angle xcex8 at which light is incident on the diffraction grating, both of which are schematically shown in FIG. 9E, are progressively reduced, the resonance and selected wavelengths shift toward a short wavelength side.
At this point, if a difference corresponding to the half of the FSR occurs between the resonance wavelength of the oscillating mode and the selected wavelength, the oscillating wavelength shifts from the oscillating mode to the adjacent one in such a manner that the state in FIG. 9C shifts to the state in FIG. 9D.
This phenomenon is called a xe2x80x9cmode hopxe2x80x9d or xe2x80x9cmode jumpxe2x80x9d.
Thus, to continuously vary the oscillating wavelength over a wide band, the oscillating resonance wavelength and the selected wavelength are linked together, that is, in the Littrow-mounting external-resonance laser, the resonator length and the angle of the diffraction grating are simultaneously varied while being maintained in an appropriate relationship, to restrain the mode hop.
Furthermore, in the tunable laser source apparatus having the external cavity structure such as the diffraction grating, the reflectivity on the external cavity side of the LD must be reduced to restrain internal modes of the LD.
Thus, a configuration with an AR coat comprising a dielectric film, that is, a configuration such as that shown in FIG. 8A which has the AR coat applied to the end surface 51a of the LD 51 is conventionally used.
The configuration with the AR coat simply applied to the end surface 51a of the LD 51, however, provides an insufficient achievable reflectivity, and significant internal modes result from residual reflectivity.
Thus, such a configuration has the following adverse effects: the mode hop occurs as descried above, which may lead to multimode oscillation, and the variable wavelength band is insufficient, thereby increasing spontaneous emitted radiation.
It is an object of the present invention to provide a tunable laser source apparatus that is developed in view of the above problems, that can sufficiently restrain internal modes of an LD to reduce spontaneous emitted radiation in order to prevent multimode oscillation, and that can extend a wavelength varying range.
To attain the above object, according to one aspect of the present invention, there is provided a tunable laser source apparatus including an external cavity, the apparatus comprising:
a semiconductor laser including a reflection surface formed on one end, a surface with an anti-reflection film formed on the other end, and an active layer extending from the reflection surface toward the surface with the anti-reflection film; and
wavelength selecting means for selecting from laser light emitted from the semiconductor laser through the surface with the anti-reflection film and feeding laser light of a desired wavelength back to the semiconductor laser through the surface with the anti-reflection film,
wherein the semiconductor laser has a window region formed between a tip portion of the active layer extending toward the surface with the anti-reflection film and the surface with the anti-reflection film, the window region allowing the laser light of the desired wavelength fed back from the wavelength selecting means to be coupling thereon, while dilating the beam size of a portion of the laser light which is reflected from the surface with the anti-reflection film and reach the tip portion film.
Additionally, according to another aspect of the present invention, there is provided a tunable laser source apparatus including an external cavity according to, the apparatus further comprising:
angle detecting means for detecting an angle between an optical axis of light emitted from the semiconductor laser and an optical axis of diffracted light reflected from the wavelength selecting means; and
a control section for changing at least one of a resonator length of the external cavity and a selected wavelength of the wavelength selecting means so that the angle detected by the angle detecting means is zero.
Further, according to yet another aspect of the present invention, there is provided a tunable laser source apparatus including an external cavity according to, wherein the wavelength selecting means includes a diffraction grating and a reflector.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.